In recent years, as vehicular operation has become quieter, noise arising from the tire is increasingly a dominant factor contributing to automotive noise, and there is a growing demand for the reduction of it. Such tire noise includes contact frictional vibration noise occurring when the tire engages the road surface and road surface noise caused by the unevenness of the road surface. And, with passenger cars, a large proportion of the noise is pattern noise which is caused by the tread pattern formed on the tire tread surface. In particular, striking noise is caused by the impact of the discontinuous portions of tread pattern, such as lug pattern and block pattern, as they come into contact with the road surface. This noise is also called “pitch noise” because the noise typically becomes louder at a certain frequency (pattern pitch frequency) which is dependent on the pitch distance of lug grooves or blocks and the vehicular speed.
A proposed method for reducing tire noise due to impact as described above consists in employing a plurality of pitch distances so as to prevent any peaking at a single frequency.
Also, as regards lug grooves, there is a method practiced to reduce such impact noise by employing lug grooves angled with respect to the axial direction of the tire.
With tires having a block pattern, on the other hand, it is cited that axially angled lug grooves, which can change the form of each block into a parallelogram, may pose a problem of lowered rigidity of the blocks or uneven wear. Therefore, in a proposed method as illustrated in FIG. 10A, a chamfered portion 51, which is wider on the initially contacting side 50A of a block 50 and narrower in the circumferential direction of the tire and which is below the virtual contour of the tire crown, is provided on the circumferential groove side of the block 50, so that the block 50 engages the road surface gradually, thereby shifting the leading-edge and trailing-edge timings. As a result, the pitch noise is dispersed over the time axis and thus can be reduced. Also, in another proposed method as illustrated in FIG. 10B, a block 60 is provided with a flat portion 61 of a predetermined width from the leading edge 60A to the trailing edge 60B, which has a height equal to the virtual contour of the tire crown and extends at an angle with respect to the circumferential direction of the tire, and sloping portions 62, 63, which are gradually descending toward the respective circumferential grooves, on the initially contacting side 60A and the finally contacting side 60B, respectively, of the block 60. And this arrangement allows the block 60 to engage and disengage the road surface gradually, thereby shifting the leading-edge and trailing-edge timings, so that the pitch noise is reduced (see Reference 1, for instance).    Reference 1: Japanese Unexamined Patent Application Publication No. 2003-25810